1. Field of the Invention
This invention relates to fixtures of the type generally used in testing printed circuit boards. More specifically it relates to devices for mechanically engaging and disengaging such fixtures to and from spring loaded test probes which make up a probe field of such test devices.
2. Description or the Prior Art
Fixture latching mechanisms provide means for mass terminating a circuit board test fixture to a testhead. A mass electrical interconnect is achieved when the wire wrap posts in the fixture are made to contact and compress an array of spring loaded probes which form the probe field portion of the testhead.
Those skilled in this art will appreciate that even though the force produced by each individual probe is usually small (e.g., 2.5 oz. at about a working travel of about 0.07 inches) the large number of probes (e.g., hundreds and even thousands per testhead) can collectively create a large overall force which must be overcome in order to achieve and maintain proper electrical contact between all respective members of an array of wrap posts and probes. Fixture latch mechanisms are designed to provide the total force needed to compress the spring loaded probes in a precise and uniform manner.
Vacuum, electrical and mechanical methods have been used to provide the forces needed to properly contact such fixtures to their intended probe field. However, each of these methods has presented various problems. Vacuum operated systems have presented problems which generally revolve around sealing the vacuum chamber under all possible conditions of operation. For example, since such probes are usually also mounted on cards, each of the many individual cards employed has to be sealed at the top of the testhead. Moreover, it is entirely possible that not all of the cards will be employed. Thus, filler panels are required to seal the chamber where the cards are absent.
Another problem arises when the interface is required to accommodate both large and small fixtures. In such a case two vacuum chambers are required with valving between them. All such sealing problems ultimately add cost and complexity; they also generally contribute to reduced reliability. Some vacuum sealing problems can be eliminated by removing the probes from the board and putting them directly in the testhead. However, this arrangement creates an added interface which tends to degrade electrical performance. It also increases cost by forcing the user to buy the maximum number of probes when he may only need a small fraction of the total to meet his particular testing needs. This added cost is significant because even though individual probe cost is relatively small the quantity is large (several thousand per testhead is not at all unusual) and therefore so is the total cost.
Various mechanical linkage systems also have been employed in such fixture latching mechanisms with varying degrees of success. One such system utilizes a bullet nosed cylindrical contact which resides in the fixture and which mates with a socket in the testhead to complete the electrical connection. The fixture is brought into contact with the testhead by a hand crank actuated cam mechanism. The main problems associated with this system is damage to the bullet nosed contact as the fixture is inserted and removed from the test device. Costs and the use of manually actuated engaging operations also present problems. This follows from the fact that while the cost of the socket in the testhead is relatively modest per contact, the cost of the bullet nose contact is rather high per contact. Since the average user must buy many fixtures, it is not unusual for the fixture cost to exceed the system cost. Moreover, the crank actuated cam mechanism used in many designs currently in use is not only costly it also has the disadvantage of requiring the operator to manually turn a crank to actuate the mechanism. Many operators of small physical stature have had difficulty supplying the required force to actuate such hand operated mechanisms over a work day.
Another manually operated fixture testing system is disclosed in U.S. Pat. No. 4,230,985. It has a fixture latching mechanism wherein a series of mating slots are provided in a configuration matching a row of mating guides on the fixture. The mating guides are inserted into mating slots. When a handle is manually turned, a track is drawn horizontally to force the mating guides, and their associated fixture, downward and into contact with an array of spring loaded probes. Because this system uses such a large number of mating guides and slots which must be simultaneously aligned, lowered and engaged by a horizontally moving rail, it takes a great deal of time to change fixtures and/or carry out circuit board testing operations. Moreover, since the mating guides project from the fixture they are apt to be bent or broken during the frequent fixture changing operations implicit in the use of such test devices.
Yet another mechanical system for providing a mass electrical interconnect involves the use of zero insertion force connectors or "ZIFs". In this design a latching mechanism is not required because no force is required to insert the connectors although there is some force required to actuate the ZIF. The main disadvantage of the ZIFs is that they usually require long wire lengths which degrade electrical performance to such an extent that such systems have not, to date, proven to be reliable enough to gain widespread acceptance.
The problems associated with the above noted electrical air and mechanical systems suggest the need for a simpler, more reliable, faster operating fixture latching mechanism whose engaging guides are withdrawn to safe unexposed positions during fixture changing operations.